The goal of this work is to study the structure and function of the immunoglobulin molecule. The work is being performed so that this understanding will be applied to the development of antibodies for human therapy. A. Therapeutic systems. 1. Antibodies to surface antigens of group B streptococci (GBS) have been demonstrated to have protective efficacy in a model of neonatal sepsis. We have identified colony opacity variants of GBS and studied their interactions with antibodies and other components of the immune system. 2. The efficacy of anti-HIV antibodies coupled to ricin A-chain has been studied in vitro. Monoclonal and polyclonal antibodies directed against different epitopes have been tested. Biological variants of HIV that escape killing with these immunotoxins and CD4-PE40 have been identified. The phenotype of the cells carrying these HIV has been studied, and the molecular mechanisms of immunotoxin escape have been evaluated. We have also constructed anti-MuLV immunotoxins, and the efficacy of anti-retroviral immunotoxins is being tested in vivo in well-studied animal systems. B. Genetically engineered antibodies. Vectors have been prepared carrying human constant region genes. These constructs contain both native and altered genes. These constructs have been ligated to variable region genes from antibodies to the synthetic polypeptide (Tyr,Glu)-Ala--Lys. Antibodies were expressed and purified. The antigen-binding capability of these antibodies has been tested. Well- defined immune complexes were constructed using antigens having different epitope densities. The ability of the immune complexes to interact with the complement system has being studied.